Metformin associated lactic acidosis in Auckland City Hospital 2005 to 2009.

AIM: To determine the incidence, clinical characteristics and outcomes of patients with metformin associated lactic acidosis (MALA).

METHODS: Auckland City Hospital drains a population of just over 400000 people. All cases presenting with metabolic acidosis between July 2005 and July 2009 were identified using clinical coding. A retrospective case notes review identified patients with MALA. Prescribing data for metformin was obtained from the national pharmaceutical prescribing scheme.

RESULTS: There were 42 cases of metabolic lactic acidosis over 1718000 patient years. There were 51000 patient years of metformin prescribed to patients over the study period. There were thirty two cases of lactic acidosis due to sepsis, seven in patients treated with metformin. Ten cases of MALA were identified. The incidence of MALA was estimated at 19.46 per 100000 patient year exposure to metformin. The relative risk of lactic acidosis in patients on metformin was 13.53 (95%CI: 7.88-21.66) compared to the general population. The mean age of patients with MALA was 63 years, range 40-83 years. A baseline estimated glomerular filtration rate was obtained in all patients and ranged from 23-130 mL/min per 1.73 m(2). Only two patients had chronic kidney disease G4. Three patients required treatment with haemodialysis. Two patients died.

CONCLUSION: Lactic acidosis is an uncommon but significant complication of use of metformin which carries a high risk of morbidity.